A typical cigarette consists of a tobacco rod wrapped with a cigarette paper. In many cases, cigarettes are also equipped with a filter, typically made from cellulose acetate, which is wrapped by a filter wrapping paper and is additionally wrapped on the outside by a tipping paper, which is slightly longer than the filter and thus connects the filter to the tobacco rod wrapped with the cigarette paper. Such cigarettes are usually consumed by burning the tobacco and the smoke created thereby is inhaled by the smoker through the filter.
Alternative smoking articles do not burn the tobacco but just heat it, whereupon an aerosol is released which is inhaled by the smoker. It is assumed that the aerosol of such smoking articles contains fewer harmful substances than the smoke of conventional cigarettes. Instead of tobacco, other aerosol-generating substances can also be used. Depending on the construction of these smoking articles, a wrapping paper can also be required for such smoking articles which wraps the tobacco or the aerosol-generating substance or other parts of the smoking article.
When designing wrapping paper for smoking articles, the optical properties of the wrapping paper play an important role in addition to the technical requirements. Because advertising for smoking articles, in particular for cigarettes, is restricted or widely prohibited in many countries, one way round this consists in distinguishing the smoking article in the perception of the consumer by the design of the smoking article itself. Thus, the wrapping paper of the smoking article is an important tool, because in conventional smoking articles, it forms most of the outer surface of the smoking article. Typical properties of the wrapping paper which are relevant to distinguishing the smoking article are whiteness, transparency or opacity, gloss, water marks, vergé lines or patterns such as logos or text applied to the wrapping paper, for example by printing. The appearance of the ash after consumption of the smoking article can also play a role.
An essential feature of the wrapping paper for the differentiation of the smoking article is its transparency, i.e. the ability to let light shine through it. An increased transparency means that more light passes through the wrapping paper. On smoking articles, the regions of higher transparency on the wrapping paper usually appear darker, as the tobacco or the aerosol-generating material located under the wrapping paper shines through.
A typical wrapping paper for smoking articles comprises pulp fibers, for example wood pulp or flax pulp, and one or more filler materials, for example calcium carbonate. Wrapping papers without filler materials are comparatively transparent, while the transparency decreases with increasing filler content. The selection of the filler can also influence the transparency. In particular, titanium dioxide as a filler can significantly decrease the transparency. In conventional production processes for wrapping papers for smoking articles, the transparency of the wrapping paper can be influenced by the composition of the wrapping paper as a whole, but not in regions.
To distinguish a smoking article, it may be desirable for the wrapping paper to contain regions of higher and lower transparency. For that purpose, various methods are available in the prior art which, however, are not without disadvantages.
A method known in the prior art is to compress the wrapping paper in certain regions. Because of the lower thickness and the denser paper structure, the wrapping paper becomes more transparent in the compressed regions. In this manner, for example, water marks or so called vergé lines can be produced. Vergé lines are narrow lines along which the wrapping paper is compressed, so that they can form darker lines on the smoking article in the circumferential or the longitudinal direction. Any pattern can be produced in the wrapping paper using this same conventional method for the production of vergé lines.
While the production of water marks or vergé lines is usually carried out during the production of the wrapping paper on the paper machine, as an alternative method, the wrapping paper can also be embossed after production on the paper machine. Here again, the wrapping paper is compressed and as regards the optical appearance and the influence on other properties of the wrapping paper, the method is similar to the production of water marks or vergé lines.
A technical disadvantage of both methods is that the compression substantially reduces the air permeability of the wrapping paper. The air permeability of the wrapping paper allows the access of air from the outside through the wrapping paper into the smoking article. In this manner, the smoke or the aerosol is diluted and the amount of potentially harmful substances in the smoke or the aerosol is reduced. A reduction in the air permeability of the wrapping paper in the compressed regions is thus generally a disadvantage.
A further disadvantage of the known methods is that the compression reduces the thickness of the wrapping paper in some regions. The surface of the wrapping paper thus becomes rougher and compromises the haptic qualities of the wrapping paper.
Finally, the known methods have the further disadvantage that in the tensile strength of the wrapping paper is reduced the compressed regions. In particular, if the compressed regions extend transversely across the wrapping paper, they create weak points in machine direction, which may cause tearing of the wrapping paper during further processing.
A further method may be to use a wrapping paper which is translucent over its entire surface as a starting point and to print it in regions so that it becomes less transparent in these regions. However, to accomplish this, compositions which contain at least one pigment or a colorant and at least one binder have to be applied to these regions.
In many cases, colorants are prohibited by law for use in wrapping papers for smoking articles, so that generally, only pigments can be used. In both cases, however, a binder is required, which needs to be applied to the wrapping paper in a comparatively large amount in order to fix the pigments or colorants to the wrapping paper. However, this binder closes the pore structure of the wrapping paper and reduces the air permeability even more than is the case for embossing. Thus, the amount of carbon monoxide and other harmful substances in the smoke is increased, which is not desirable.
Thus, there is a need to have a wrapping paper available which has an increased transparency in certain regions, but which does not change substantially in other properties such as air permeability, thickness and tensile strength.